Iranians wave their national flag as they hold a poster of President Hassan Rouhani, while welcoming Iranian nuclear negotiators upon their arrival from Geneva at the Mehrabad airport in Tehran, Iran, in November 2013. Despite their long confrontational history, the US and Iran have increasingly found themselves on the same side on regional issues, particularly in regard to fighting Sunni militants in the Middle East.

The Geneva II conference is set to begin Jan. 22, and will include more than 20 countries invited by the UN as well as representatives from Syria’s opposition. The UN special envoy to Syria, Lakhdar Brahimi, supports Iran’s participation in the Syrian peace process, and on Sunday, US Secretary of State John Kerry said Tehran could “participate very easily” in the talks if they accept that the Assad regime must be replaced by a transitional government.

"If Iran doesn't support that, it's difficult to see how they are going to be a ministerial partner in the process," Mr. Kerry said, noting that there are ways they could “conceivably” contribute from the sidelines.

Since its 1979 Islamic revolution, Iran has used Syria as a conduit for weapons, cash, and support for the Lebanese Shiite militant group Hezbollah, and later Hamas and Islamic Jihad, all of which form a frontline against Israel. If Assad falls, Iran could lose that channel.

Reuters reports that the US State Department spokesman said that for Iran to have a role in the Syria peace talks, “they would have to demonstrate that they would do things that would be less destructive in Syria."

The New York Times reports that while the US and Iran “quietly continue to pursue their often conflicting interests, they are being drawn together by their mutual opposition to an international movement of young Sunni fighters, who with their pickup trucks and Kalashnikovs are raising the black flag of Al Qaeda along sectarian fault lines in Syria, Lebanon, Iraq, Afghanistan and Yemen.” On Monday, Iran offered to join the US in sending military aid to the Iraqi government, which is engaged in a fierce struggle to oust Sunni militants from Iraq's Anbar province.

With Iran as an island of stability in a region plagued by violent protests, sectarian clashes and suicide bombers, there are not that many options left for Washington, experts here say.

“We face the same enemy, and the enemy of my enemy is my friend,” said Mashallah Shamsolvaezin, a prominent Iranian reformist journalist who closely follows the Arab world. He recalled how Iranian intelligence operatives gave reliable information to American Special Forces troops battling Iran’s enemy, the Afghan Taliban, in 2001.

While the Obama administration acknowledges that Iran has the potential to be an influential player on regional issues from Afghanistan to Syria, senior officials have said they are keeping their focus tightly on the nuclear negotiations. Cooperation on any other issues, they said, hinges largely on coming to terms on Iran’s nuclear program.

On Monday, an unnamed senior State Department official told reporters that “There are ... steps that Iran could take to show the international community that they are serious about playing a positive role [in Syria]."

"Those include calling for an end to the bombardment by the Syrian regime of their own people. It includes calling for and encouraging humanitarian access." The official told Reuters that Iran hadn’t shown any evidence of taking these types of steps.

“The Americans are confessing Iran stands for peace and stability in this region,” Hamid Reza Tarraghi, a hard-line political analyst told The New York Times. “But when they invite us for a conference on Syria we are ‘allowed’ to be present on the ‘sidelines.’ This is insulting.”